Abstract: A large signal self-consistent solution of orotron for a simplified model is given. In this model the electron beam is modulated by the field and the field is excited by the electron beam inversely. A parameter Zc, which is very important for the efficiency (as well as for getting oscillation), is deduced. By means of this simplified model calculation is carried out for some practical parameters of orotron. It is shown that for producing a practical orotroa in wide frequency range, it is necessary to design a quasi-optical resonator not only with a high QH but also with a wide and long enough mode spot on the grating. And for the electron beam, a very small thickness is needed and it ought to pass by the grating as closely as possible, and a high enough value of current is also very necessary.